The field of transport has several aspects: loosely they can be divided into a triad of infrastructure, vehicles, and operations. Infrastructure includes the transport networks (roads, railways, airways, canals, pipelines, etc.) that are used, as well as the nodes or terminals (such as airports, railway stations, bus stations and seaports). The vehicles generally ride on the networks, such as automobiles, trains, airplanes. The operations deal with the control of the system, such as traffic signals and ramp meters, railroad switches, air traffic control, etc, as well as policies, such as how to finance the system (e.g. use of tolls or gasoline taxes in the case of highway transport).
Broadly speaking, the design of networks are the domain of civil engineering and urban planning, the design of vehicles of mechanical engineering and specialized subfields such as nautical engineering and aerospace engineering, and the operations are usually specialized, though might appropriately belong to operations research or systems engineering.
Modes of transport
Modes are combinations of networks, vehicles, and operations, and include walking, the road transport system, rail transport, ship transport and modern aviation.
Categories of transport
* (Non-human) Animal-powered transport
Transport and communications
Transport and communication are both substitutes and complements. Though it might be possible that sufficiently advanced communication could substitute for transport, one could telegraph, telephone, fax, or email a customer rather than visiting them in person, it has been found that those modes of communication in fact generate more total interactions, including interpersonal interactions. The growth in transport would be impossible without communication, which is vital for advanced transportation systems, from railroads which want to run trains in two directions on a single track, to air traffic control which requires knowing the location of aircraft in the sky. Thus, it has been found that the increase of one generally leads to more of the other.
Transport, activities and land use
There is a well-known relationship between the density of development, and types of transportation. Density is defined as area of floorspace per area of land. As a rule of thumb, densities of 1.5 or less are well suited to automobiles. Densities of six and above are well suited to trains. The range of densities from about two up to about four is not well served by conventional public or private transport. Many cities have grown into these densities, and are suffering traffic problems. Personal rapid transit might fill this gap.
Land uses support activities. Those activities are spatially separated. People need transport to go from one to the other (from home to work to shop back to home for instance). Transport is a "derived demand," in that transport is unnecessary but for the activities pursued at the ends of trips. Good land use keeps common activities close (e.g. housing and food shopping), and places higher-density development closer to transportation lines and hubs. Poor land use concentrates activities (such as jobs) far from other destinations (such as housing and shopping).
There are economies of agglomeration. Beyond transportation some land uses are more efficient when clustered. Transportation facilities consume land, and in cities, pavement (devoted to streets and parking) can easily exceed 20% of the total land use. An efficient transport system can reduce land waste.
Transport, energy, and the environment
Transport is a major use of energy. Most transport burns hydrocarbons. If partially burned, these create pollution. Though vehicles in the United States have been getting cleaner because of environmental regulations, this has been offset by more vehicles and more use of each vehicle. Low-pollution fuels can reduce pollution. The most popular low-pollution fuel at this time is liquified natural gas. Hydrogen is an even lower-pollution fuel, but producing and storing it economically is currently not feasible. Other alternative renewable energy sources such as biodiesel are being researched heavily.
Another tack is to make vehicles more efficient, which reduces pollution and waste by reducing the energy use. If electricity can be gotten to the vehicle, electric motors are the most efficient of all. Another method is to generate energy using fuel cells, which are two to five times as efficient as the heat engines traditionally used in vehicles. A trivial, but very effective method is to streamline ground vehicles, which spend up to 75% of their energy on air-resistance. Another method is to recycle the energy normally lost to braking, but this leads to a more complex vehicle.